1. Field of the Invention
The present invention relates to acoustic wave devices and filters, and more particularly, to an acoustic wave device having a capacitance formed by electrodes that horizontally face each other on a piezoelectric substrate and a filter using the same.
2. Description of the Related Art
A surface acoustic wave (SAW) device is well known as one of acoustic wave devices that utilize acoustic waves. The SAW device has comb electrodes formed by an interdigital transducer (IDT) formed on a surface of a piezoelectric substrate. Electric power is applied to the comb electrodes, and an acoustic wave is excited. The SAW device can be miniaturized and lightened in weight. Further, the SAW device is capable of realizing high attenuation. Because of these advantages, the SAW filter is widely applied to devices processing radio signals in a frequency range of, for example, 45 MHz to 2 GHz, such as transmission bandpass filters, reception bandpass filters and antenna duplexers.
There is another acoustic wave device called acoustic boundary wave device in which an acoustic wave is propagated along a boundary between two different media. This type of device has an advantage in that even if foreign particles adhere to surfaces of the two different media, these particles do not cause frequency variation an loss, so that downsizing can be easily realized. There is another advantage in that packaging does not need a hermetically sealed hollow space structure. It is to be noted that the SAW device is required to be mounted in the hermetically sealed hollow space structure in order to prevent foreign particles from adhering to the surface of the device. Furthermore, the temperature coefficient of frequency can be improved by covering the comb electrodes with an SiO2 film.
The recent sophisticated performance of cellular phones needs acoustic wave devices having improved performance such as lower loss in the pass band and greater attenuation in stop bands, and downsizing. For example, in the PCS (Personal Communication Service) system that is one of mobile phone service systems in the North America, the pass band and the stop band are very close to each other. It is well known that a material having a small electromechanical coupling coefficient (k2) is advantageously used to realize the filter having the pass band and the stop band close to each other. However, k2 is the physical factor inherent in material itself and has a specific value that depends on the selected material. For example, k2 of 42° Y-cut X-propagation lithium tantalate (LiTaO3) widely used for bandpass filters in cellular phones is approximately equal to 7%.
It is very difficult to control the value of k2 itself. Thus, there have been proposals for effectively reducing k2. FIG. 1A is a plan view of a conventional SAW device, and FIG. 1B is an equivalent circuit diagram of the SAW device shown in FIG. 1A. Referring to FIG. 1A, there is illustrated a resonator 13 in which a pair of reflection electrodes R1 and a pair of comb electrodes 12 interposed between the reflection electrodes R1 are formed on a piezoelectric substrate 16. A capacitor 14 composed f a pair of comb electrodes is formed on the piezoelectric substrate 16. The capacitor 14 is connected in parallel with the pair of comb electrodes 12, and has a different period from that of the comb electrodes 12. The pair of comb electrodes 12 of the comb-electrode type capacitor 14 is composed of comb electrodes 14a an 14b, which horizontally face each other on the piezoelectric substrate 16. A desired resonance frequency can be obtained by connecting the comb-electrode type capacitor 14 to the resonator 13. Thus, the value of k2 can be effectively controlled. It is to be noted that only a few electrode fingers of the comb electrodes 12, the reflection electrodes R1 and the capacitor 14 are illustrated for the sake of simplicity. However, actually, a large number of electrode fingers is provided.
In the SAW device shown in FIGS. 1A and 1B, the comb-electrode type capacitor 14 has a large value of the resonance sharpness (quality factor Q) in a frequency range lower than the resonance frequency of the capacitor 14. However, only a small value of the resonance sharpness Q is available in a frequency range higher than the resonance frequency. As the resonance sharpness Q has a larger value, the device has a smaller insertion loss. It is therefore desired that the Q value of the comb-electrode type capacitor 14 is as large as possible.